CDMA is a multiple access method based on a spread spectrum technique, and it has been recently put into use in cellular radio systems in addition to previously used FDMA and TDMA. CDMA has many advantages over the prior methods, such as simplicity of frequency planning, and spectrum efficiency.
In a CDMA method, a narrow-band data signal of a user is multiplied to a relatively broad band by a spreading code having a much broader band than the data signal. Band widths used in known test systems include e.g. 1.25 MHz, 10 MHz and 25 MHz. The multiplication spreads the data signal over the entire band to be used. All the users transmit simultaneously on the same frequency band. On each connection between a base station and a mobile station is used a different spreading code, and the signals of the users can be distinguished from one another in the receivers on the basis of the spreading code of the user. If possible, the spreading codes are selected in such a way that they are mutually orthogonal, i.e. they do not correlate with one another.
Correlators in conventionally implemented CDMA receivers are synchronized with a desired signal, which they recognize on the basis of the spreading code. In the receiver the data signal is restored to the original band by multiplying it by the same spreading code as in the transmission step. Ideally, the signals that have been multiplied by some other spreading code do not correlate and are not restored to the narrow band. In view of the desired signal, they thus appear as noise. The object is to detect the signal of the desired user from among a number of interfering signals. In practice, the spreading codes correlate, and the signals of the other users make it more difficult to detect the desired signal by distorting the received signal. This interference caused by the users to one another is called multiple access interference.
The more simultaneous users there are in the system, the greater the multiple access interference. Thus the capacity of the CDMA cellular radio system is limited by the above-described interference that the users cause to one another. Interference can be reduced by maintaining, if possible, the power levels of the signals transmitted from the terminal equipments as equal as possible in the base station receiver by means of accurate power adjustment. The base station observes the power levels of the received signals and transmits power adjustment commands to terminal equipments. Another known way of reducing interference is to use different active methods of eliminating multiple access interference and methods based on simultaneous detection of a plural number of users.
The essential feature in a multipath channel is that the complex amplitude and the delay of each received signal component having a significant power level can be estimated so that the corresponding interference component can be eliminated from the desired signal. This determination of the amplitude and delay is called channel estimation.
In the prior art solutions of channel estimation, elimination of interference has not been taken into account but these two actions have been considered as mutually independent solutions. Due to this, the estimation results have been based on a signal that comprises interfering signals from a plural number of users, and so the estimation results are not ideal.